124 NATURAL SELECTION. Chap. IV. 



more parent-species are supposed to have descended 

 from some one species of an earlier genus. In our 

 diagram, this is indicated by the broken lines, beneath 

 the capital letters, converging in sub-branches down- 

 wards towards a single point; this point representing 

 a single species, the supposed single parent of our 

 several new sub-genera and genera. 



It is worth while to reflect for a moment on the cha- 

 racter of the new species f u , which is supposed not to 

 have diverged much in character, but to have retained 

 the form of (F), either unaltered or altered only in a 

 slight degree. In this case, its affinities to the other 

 fourteen new species will be of a curious and circuitous 

 nature. Having descended from a form which stood 

 between the two parent-species (A) and (I), now sup- 

 posed to be extinct and unknown, it will be in some 

 degree intermediate in character between the two 

 groups descended from these species. But as these two 

 groups have gone on diverging in character from the 

 type of their parents, the new species (f u ) will not be 

 directly intermediate between them, but rather between 

 types of the two groups ; and every naturalist will be 

 able to bring some such case before his mind. 



In the diagram, each horizontal line has hitherto been 

 supposed to represent a thousand generations, but each 

 may represent a million or hundred million generations, 

 and likewise a section of the successive strata of the 

 earth's crust including extinct remains. We shall, when 

 we come to our chapter on Geology, have to refer again 

 to this subject, and I think we shall then see that the 

 diagram throws light on the affinities of extinct beings, 

 which, though generally belonging to the same orders, 

 or families, or genera, with those now living, yet are 

 often, in some degree, intermediate in character between 

 existing groups ; and we can understand this fact, for 



